Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

X-ray spectra

  1. Jun 8, 2005 #1
    just been learning how to produce x-rays by firing electrons at heavy atoms, but i am a tad confused about the x-ray spectra graph with 'intensity' on the y and 'wavelength' on the x. it shows that the minimum wavelenth is produced wen intensity=0. how is this possible?
  2. jcsd
  3. Jun 8, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    The details of the spectrum are complicated, so this is not a full explanation why the intensity spectrum looks the way it does. It will help you though to understand that the intensity at any wavelength depends on both the energy of the individual photons, and the number of photons produced at that energy. A detailed examination of the collisions of electrons with heavy nucleii will show that the kind of collision that results in the shortest wavelength photons is far less likely than collisions that produce longer wavelengths. The longer wavelength photons are produced in greater numbers, but the intensity gradually decreases because of the lower energy per photon. There is no limit on how long the wavelength can be, which is why there are those long tails on the right side of the spectrum curve. There is a limit on how short the wavelength can be, depending on the initial velocity (or kinetic energy) of the electron. That is why the curves show a lower left end of the spectrum as the electron energy increases.

  4. Jun 9, 2005 #3
    the x-ray spectrum is just a histogram showing the distribution of the various x-ray energies in the beam, not what x-ray energies/wavelengths are produced at particular intensities. All the spectrum tells you is that for that particular beam, at a wavelength of x nm or x eV, you will see an intensity of y. At the minimum wavelength (maximum x-ray energy), the intensity of the beam is 0 (none or very little).
  5. Jun 9, 2005 #4

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    Just to underscore what has already been said, the minimum wavelength (highest energy) photon is created in the rare event when ALL the kinetic energy of an elecron is converted into a photon. At this point the intensity is "almost zero." Since two electrons can't "gang up" their energies, no photons of higher energy than this can be created.

    According to my read of your question, this may be your concern:
    The point on the graph you are looking at for "minimum wavelength" is actually an infinitessimally small increment to the right of the "zero" intensity point. For all practical purposes, you just read the "zero" point.
  6. Jun 9, 2005 #5
    ok thanks everyone. Think i somehow confused myself into thinking that intensity reffered 2 the bombarding electrons.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook